Semiconductor device, semiconductor layer and production method thereof

A method for preparing a semiconductor layer, comprising a boron phosphide-base mixed crystal which contains boron phosphide, the boron phosphide having a band gap of not less than 2.8 eV and not more than 3.4 eV at room temperature and which mixed crystal is represented by the formula:

• BαAlβGaγIn1-α-β-γPδAs∈N1-δ-∈ (0<α≦1, 0≦β<1, 0≦γ<1, 0<α+β+γ≦1, 0≦δ≦1, 0≦∈<1, 0<δ+∈<1, 0<∈α∈≦1),
• which method comprises forming a semiconductor layer by a metallo-organic chemical vapor deposition method (MOCVD method), at a temperature higher than 750° C. and not higher than 1200° C., with a ratio of the total supply amount of Group V element sources including phosphorus (P) to the total supply amount of Group III element sources including boron (B) being not less than 15 and not more than 60 and the growth rate of the semiconductor layer being not less than 2 nm/min, and not more than 30 nm/min.